Environmental lightingpowerfully influencesthe circadian system in humans. Particularly, light exposure that occurs at night may have an adverse effect on cancer risk through suppression of melatonin, a hormone that is intimatelylinked with the circadian system and which may have cancer protective effects. In observational studies, night work has consistently been associated with an increase in breast and colorectal cancer risk, potentially mediated through the melatonin pathway. In 2003, the Institute of Cancer Research for the Health and Safety Executive (HSE) classified the overall evidence for an association between shift work and risk of breast cancer appreciable. Up to 17 percent of all full-time wage and salary workers work alternative shifts in the U.S. Among those, shifts that include night work are most prevalent and account for 6.4 percent of all full-time wage and salary workers. Although much work has been done to measure the effects of light on melatonin,light measurements, particularlyin the short wavelength portion of the visible spectrum, which only recently was found to have the largest impact on circadian functions in humans, were not able to capture the precise effects of light on melatonin, to date. Thus, this proposal will evaluate light exposure in rotating shift workers based on the recent information on the specific wavelengths that affect melatonin levels in humans, and will relate those measures to the response of their circadian system, as measured by melatonin levels in urine. Specifically, we will measure light exposure by applyinga new device, a circadian light meter that captures the short wavelength portion of visible light, which has been described to most strongly suppress melatonin levels and phase shift the circadian pace maker as part of the light's influence on non-image forming function in humans. Urine collections will be conducted duringthree of the overall 7 days of study period. The studyinvolves approximately 180 women, all participants of the ongoing Nurses Health Study II. By selectinga sample of women stratified on their shift schedules we will be able to evaluate differential effects of light exposure in subgroups of night workers. Because the proposed study takes advantage of previously collected detailed demographic and medical information in a large prospective study of nurses, the NHS II cohort, the proposed study is bot!i a particularly relevant contribution to current literature and also very cost-effective.